Stabilisation buoys used in particular by autonomous divers are known which, when they change depth during diving, must compensate manually for the volume of their buoy, either by injecting air originating from the first-stage expander of their gas bottle, or by purging the buoy by acting on purge valves located at different places in the latter; this is necessary at each change in depth as the volume of air/gas of the buoy is modified with the variations in pressure associated with this change in depth and must therefore be adjusted permanently or otherwise not allow the diver to stabilise at the desired depth.
In addition, since such a buoy is made from a supple bladder generally made of polyurethane, the air/gas is always located in its upper part, which makes movements by the diver carrying said buoy difficult and, when the latter is deflated, its form is generally not hydrodynamic.
The problem posed is therefore to produce an adjustable floatability device in a predetermined range, at a preferred volume value which must then remain constant during change in depth without intervention of the operator, offering the latter a constant reserve floatability, irrespective of the ascending and descending shifts of the submersible entity to which the device is attached; and when this entity is the diver operator himself, this device must allow him ease of movement and must have the most hydrodynamic form possible.
A solution to the problem raised is a floatability device having constant and adjustable volume designed for any submersible entity, capable of immersing to different depths, and comprising at least one float with a supple membrane filled with gas and whereof the pressure is regulated as a function of the immersion depth, and such that                this membrane is elastic and delimits a gas volume which is a function of the elasticity of this membrane and of the relative internal pressure of said volume in relation to the external ambient pressure,        an expander-discharge system ensuring supply and purging of said gas to or from said internal volume of the float.        